Independent Front Suspension (IFS) sometimes gets a bum rap in the off-road world. Common wisdom dictates that IFS is good for high-speed running and bouncing down rough backroads, but the solid axle typically gets the nod of approval for hardcore off-roading with big tires.
There are certainly applications in which using IFS is a good thing and can yield tractable handling and a good ride. But can the differentials take some of the abuse we dish out and survive when running larger, heavier tires?
As you modify the driveline to accommodate bigger meats with more rolling mass, you may start to upgrade differential components and CV axle shafts. But, at some point you may simply overstress the housing, especially if it is an aluminum one.
Also, over the years we’ve seen people stuff a number of upgraded center sections into the front crossmember area of an IFS truck. However, these conversions typically require a fair bit of custom fabrication to get the new center chunk situated under the engine, and incorporate it and upgraded axles with the A-arms and other components.
So, with an eye towards exploiting the IFS (or IRS) side of the hardware, what are some of the more noteworthy independent differentials and their characteristics? Let’s take a look:
The GM 9.25-inch is the beefiest GM front chunk and has been used on a large number of GM fullsize vehicles from about 1988 to 2010, including 1/2- to 1-ton truck lines, Tahoes, Yukons, Denalis, Suburbans, and in the Hummer H2. The aluminum housing holds a ring gear with diameter of 9.25 inches and the gear is secured with 12 bolts. Axle spline count is 33 splines. A single carrier is designed to accommodate axle ratios from 3.42:1 to 5.38:1.
H3 Iron Diff
The General Motors H3 (starting in 2009) uses a cast-iron front housing similar to the aluminum version shown here. Both use a 7.63-inch ring gear. The axleshafts are 26-spline units and there are 10 bolts securing the ring gear to the carrier. A single carrier is designed to accommodate all axle ratios. We spoke with famed racer Rod Hall who has extensive competition experience with the Hummer line and he commented that the iron housing version in the late model Hummer H3 has proven to be sturdy and reliable. The early aluminum housings were prone to flexing and would sometimes fracture the mount ears. Experience with the aluminum H2 ones showed they would often snap under stress when the front axle was in high lock, so the team got in the habit of often running with the front unlocked when they could.
The Toyota SD223AD has been used in the front axle of 2007-to-present Tundras and 2008-and-newer Sequoias. It uses an 8.7-inch ring gear held with 10 bolts. The differential housing is made from aluminum alloy and a single carrier is designed to accommodate all axle ratios. Axle spline count is 34 splines.
WK Grand Cherokee IRS
The 2011 and newer model Jeep Grand Cherokee (WK) comes with IRS in several flavors, depending on engine and drivetrain configuration. They use an 8.9-inch ring gear in either aluminum or cast iron housing. Designators are: 195RIA (rear independent aluminum), 215RII (rear independent iron), 225RII (rear independent iron), and 230RIA (rear independent aluminum; with or without ELSD)
Ford Dana 50 TTB
The Ford Dana 50 Twin Traction Beam (TTB) was used in 4WDs from 1980 to 1998 in F-250 models and from 1980 to 1986 in F-350 models. The 9-inch ring gear is held to the differential using 10 bolts. Axle spline count is 30 and a single carrier is designed to accommodate all axle ratios. This IFS style housing is of cast iron construction.
Toyota 8-Inch IFS
The Toyota 8-inch IFS is found in 2003-and-newer 4Runner, 2004-and-newer T-100 and Tacoma, and FJ Cruiser models. The differential accepts 30-spline axleshafts and there are 10 ring gear bolts. One carrier style is used for 3.91 and up ratios and another carrier is used for 3.73 and down ratios.
Ford Expedition Rear IRS
Ford used cast-iron Independent Rear Suspension (IRS) units in 2002-to-2010 Expeditions, Explorers, Lincoln Mountaineers and Navigators. These have an 8.8-inch ring gear held in place with 10 bolts. Spline count is 31.
The Currie F9 9-inch independent centersection is an example of an aftermarket housing offering with axles for use with Series 30 CVs. It’s suitable for buggy and race car applications and can be used in the front or rear. The housings are setup with third-member studs, fill cap, drain plug, and vent. The 300M axles accept Series 30 CVs, and include the bearings and retaining hardware. The housings can also accept 10-inch ring gears with some minor trimming of the third-member mounting face. Unit width is 14.5 inches from axle face to axle face. F9 images courtesy of Currie Enterprises.